Identification and pathogenicity of Aeromonas sobria on tail-rot disease in juvenile tilapia Oreochromis niloticus

Thirty-six strains, numbered from PY01 to PY36, were isolated from six moribund Oreochromis niloticus. The biochemical characteristics of all strains conformed to the species description of Aeromonas sobria on the basis of API 20E and Biolog GN system. Furthermore, gyrB sequence of strain PY36 was sequenced and showed high similarity (99.8%) with A. sobria in Genbank. Antibiotic-resistance of strain PY36 was assessed by the Kirby-Bauer disk diffusion method, and the results showed it was susceptible and moderately susceptible to 12 and 3 of the 19 antimicrobials tested. Virulence of strain PY36 to juvenile tilapia was also tested, and we found that LD₅₀ was about 4.17 × 10³ CFU per fish in intraperitoneal injection. This is the first article to report that A. sobria was the pathogenic agent of tail-rot disease in juvenile tilapia. A. sobria was multi-resistant to the most frequently used antimicrobial drugs in China, so the antimicrobial resistance test should be carried out when these bacteria are isolated from biological samples in order to avoid therapeutic failures and spread of the pathogenic organisms in the environment.